Abstract：The morphology, size, distribution and chemical composition of precipitates and the microstructure of the ultra-low carbon bainitic steel with three different nitrogen contents were investigated by optical microscope (OM), scanning electron microscope(SEM), transmission electron microscope(TEM) and energy disperse spectroscopy (EDS). The results show that the microstructure of the steel with low nitrogen content is granular bainite, while the steel with high nitrogen content is granular bainite as well as some acicular ferrite. When the value of V/N is 3.4, the yield strength and ultimate strength increase by 231MPa and 95MPa, respectively, which is attributed to precipitation strengthening and fine-grain strengthening. Compared with the low nitrogen steel, the high nitrogen steel can obtain finer bainitic ferrite lathes, the number of precipitates increases and the size decreases. There are two kinds of different sizes of the nanometer-sized precipitate particles in matrix, one of these are vanadium carbonitride, which are the range of particle sizes from 10nm to 15nm and distributed within bainitic ferrite lathes. The other precipitates are smaller than 10nm enriched with Cr and V. It seems that this carbide can maintain the NaCl-type of structure.
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